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Geometric genus of a plane curve by delta invariants

Statement

Assume the Axiom of Choice, inherited through the normalization, delta, curve-topology and cohomological genus interfaces below. Let k be algebraically closed and let F∈k[T0,T1,T2] be irreducible homogeneous of degree d≥1, defining an integral plane curve X=V+(F)⊆Pk2 whose singularities are isolated. Then the genus of the normalization is g(Xnu)=(d−1)(d−2)2−∑x∈Xδx(X), the sum running over the finitely many singular points of X.

Facts & Assumptions

Given: The Axiom of Choice and an algebraically closed field k, an irreducible homogeneous form F of degree d≥1, the integral plane curve X=V+(F) with isolated singularities, its normalization ν:Xnu→X, and the delta invariants δx(X) of its closed points.

[F1]

Under Choice, for the integral plane curve X=V+(F) of degree d one has H0(X,OX)=k and pa(X)=1−χ(OX)=(d−1)(d−2)2. (Arithmetic genus of a plane curve)

[F2]

Under Choice, for an integral proper finite-type curve over the algebraically closed field k with normalization ν:Xnu→X, one has pa(X)=g(Xnu)+∑x∈Xδx(X), the sum finite and supported on the singular points; the delta invariant is δx(X)=dim⁡k((ν∗OXnu)x/OX,x), and δx(X)=0 exactly at the regular points. (Arithmetic genus, geometric genus and delta invariants, Delta invariant of a curve singularity)

[F3]

Under Choice, for the integral plane curve X=V+(F) over the algebraically closed field k the normalization ν:Xnu→X exists, is finite and birational, and the geometric genus is defined as g(X)=g(Xnu). (Normalization of an integral finite-type curve by gluing affine integral closures, Geometric genus of a singular curve)

[F4]

Under Choice, if X is an integral finite-type k-scheme whose underlying space has chain dimension one, then a proper closed subset Z⊊X is a finite set of closed points, and every point other than the generic point is closed. (Proper closed subsets of a curve are finite)

[A1]

The Axiom of Choice is assumed for the cited interfaces. (The Axiom of Choice)

Proof

technique · direct; combine the plane arithmetic-genus computation with the normalization formula and rewrite the result as the geometric genus
1.1F2given

By [F2], each δx(X) is finite and nonnegative, vanishes at regular points, and the singular points form a finite set of closed points. Thus the correction sum is finite and has the stated support.

1.2F1

The arithmetic genus is known. Since X is an integral plane curve cut out by the irreducible form F of degree d, [F1] gives pa(X)=1−χ(OX)=(d−1)(d−2)2.

1.3F2F3

Normalization formula. Applying [F2] to X and solving for the genus of the normalization gives g(Xnu)=pa(X)−∑x∈Xδx(X); by [F3] this is the geometric genus g(X) and the normalization exists with the stated properties.

2.1A1F1F2F3F4step 1.1step 1.2step 1.3∎

Conclusion. Substituting the value pa(X)=(d−1)(d−2)2 of step 1.2 into step 1.3 gives g(Xnu)=(d−1)(d−2)2−∑xδx(X); the sum is finite by step 1.1 and vanishes exactly when X is smooth, in which case the normalization is an isomorphism and the formula recovers the plane arithmetic genus. Choice is inherited through [F1]–[F4].

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